Technical Field
Dental prostheses and apparatus and methods of manufacturing them. In particular, a method and apparatus for making a denture base disc as a starting material for a denture. The denture base disc has significantly reduced stresses and other defects and can be used advantageously in computer-implemented (CAD-CAM) methods to form a removable denture.
Description of Related Art
According to current practice, conventional dentures are made by a dental laboratory using a “lost wax” technique. In this technique, a denture base is formed out of wax on a baseplate fitted to a stone model; then plastic denture teeth are fitted into the wax. More wax is added to form interproximal (between the teeth) contours and to make the denture appear life-like. This waxed denture (known in the industry as a “wax-up”) is then delivered to a dentist for fitting into the patient's mouth. At that time, the dentist evaluates the aesthetics of the teeth of gums, and function of chewing and speech by the patient.
If adjustments are required to the wax-up, the dentist may make adjustments to the wax and/or position of the teeth, or the dentist may give instructions to the laboratory to make such adjustments. After the adjustments are made and the dentist is satisfied with the wax-up, the dental laboratory converts the wax and baseplate into a rigid methylmethacrylate plastic through a “lost wax” processing technique. This process involves the following steps:                1) A liquid slurry of a gypsum-based powder known as “dental stone” or plaster is poured into the bottom half of a metal denture flask.        2) Before the liquid dental stone begins to harden to form a solid, the lower region of the wax-up model is pushed into the liquid stone so that the liquid stone rises to the upper edge of the flask bottom half. The wax up model, which consists of the baseplate and wax denture base and the plastic denture teeth embedded in the wax base, is only immersed into the liquid stone a small distance, such that the liquid stone does not cover the teeth of the wax-up. Excess stone is removed from overflow areas on the flat edge of the flask where it will interface with an opposing flask half before the stone hardens to make it smooth and relatively flat so that no undercuts are created.        3) When the liquid dental stone hardens, its exposed top surface is coated with a separating medium such as petroleum jelly (or foil).        4) The top half of the denture flask is placed on top of the bottom half, and thus extends upwardly above the wax-up. Additional liquid dental stone slurry is poured into the top half of the flask to cover the top portion of the wax-up, including the teeth that are embedded in the wax.        5) After the dental stone has fully hardened, the flask and wax-up that is contained therein are heated to soften the wax.        6) The flask is opened, i.e., the top half and bottom half of the flask are separated. The top half of the dental flask now contains the plastic denture teeth embedded in the solid dental stone. The melted wax is washed-out with boiling water and any remaining wax is removed from the dental stone and plastic denture teeth with a brush, boiling water and soap. Sometimes, a chemical wax remover is used, followed by soap and water. Any trace of remaining wax may cause a failure of the bond between the denture teeth and the denture base. This problem is frequently the cause of denture teeth dislodging from the denture base during function (patient chewing).        7) Since the plastic denture teeth are embedded into the solid dental stone which is contained in the top half of the flask, care must be maintained not to dislodge the teeth from the stone. In order to enhance the bond between the plastic teeth and the denture base that will subsequently be formed and bonded to them, a dental laboratory technician will sometimes lightly grind the exposed surfaces of the teeth that were embedded in the wax in order to break the shiny surface (glaze) on the surfaces. In some cases, a technician will also drill very small holes into the exposed teeth bottoms (sometimes called “diatorics”) in order to allow denture base material to flow into the holes, thereby improving mechanical retention. However, the use of diatorics is risky because they often are the cause of denture tooth fracture.        8) The denture base material is then prepared and contacted with the teeth to form the denture. The most common type of denture base material used in the United States is a heat-cured methylmethacrylate material. Methylmethacrylate powder and a suitable liquid are mixed together to form a dough. The dough is placed in the void volumes of the top half of the flask which contain the artificial teeth. The void volume in the top half of the flask previously contained a portion of the wax, which was removed as described above; this top void volume now forms a mold of the upper portion of the denture base, into which the bottoms of the teeth embedded in the dental stone extend. In like manner, the void volume in the bottom half of the flask also previously contained a portion of the wax, which was removed as described above; this bottom void volume now forms a mold of the lower portion of the denture base. After placing a thin sheet of polyethylene on the bottom half, the top and bottom halves of the flask are placed together, forming a complete mold of the denture base with the dough contained therein. The flask is then placed in a press at approximately 1,000 psi to compress the dough, thereby squeezing the dough and causing it to flow into the voids that were once wax.        9) Because methylmethacrylate polymers shrink when they polymerize, it is common practice to “trial pack” the denture several times. “Trial packing” means that the denture base dough is placed in the flask and pressed, then the flask is opened again and more dough is added, then closed and pressed again. This process is repeated until the operator subjectively determines that the denture base is packed as tightly as possible. Before the last packing is done, the thin polyethylene sheet is removed from the bottom half. The last “trial pack” is typically performed at about 3,000 psi.        10) The flask then is inserted into hot water to start the polymerization process. Most heat-cured methylmethacrylate polymers are cured at about 165° F. for a few hours; then the water temperature is increased to boiling (approximately 212° F.) for at least 30 minutes. The purpose of the lower temperature cycle is to initiate polymerization. If the temperature is initially too hot, the polymer will cure too quickly, causing porosity in the denture base. The porosity is caused by rapid curing and shrinkage of the polymer before uncured polymer can fill the shrinking area. The purpose of raising the temperature at the end of the cycle is to complete the polymerization process as much as possible.        11) The last steps are to remove the denture from the flask by opening the two flask halves; then use plaster nippers or an air chisel to remove stone from the denture (devesting); then the denture base is adjusted with a high-speed bur and polished with a rag wheel and abrasives and/or polishing media.        
There are many problems with this conventional denture base fabrication method and related materials and systems:                1) Due to the many steps of processing a denture as summarized above, there are chances of introducing an error into the steps, thereby potentially increasing fit problems.        2) Tooth de-bonding from the denture base is a common problem. This is often caused by the presence of traces of remaining wax on the bottoms of the teeth, which cause a failure of the bond between the denture teeth and the denture base. Tooth fracture is also a common problem, particularly when diatorics are used in an attempt to strengthen the bond of the teeth to the denture base as described above.        3) Human errors cause inconsistencies in the denture fabrication process, which can result in defects and failures as described above, and/or poor denture fit. Examples of human errors are using different ratios of methylmethacrylate powder/liquid, using different flask closure pressures, water levels, or temperatures that vary from fabrication to fabrication. Inconsistencies can also be caused by environmental variation, such as differences in ambient temperature and/or humidity. Underpacking is an error caused by not using enough dough or too little clamping pressure. This can result in porosity (small bubbles) that may weaken the denture and make it prone to bacteria growth. Overpacking occurs when too much dough is used and then the flask halves don't fully close together and have intimate contact. This can result in mal-fitting dentures or “open bites” as is referred to by dentists. Another introduction of potential for human error is the finishing step. In the process of removing imperfections in a denture that has been devested, some dental technicians may over adjust (remove excessive denture base material), resulting in denture base that is too thin and prone to breakage. Also, the polishing step—which is the process to remove scratches created in the finishing step—a dental technician may over-polish the denture teeth, which can remove intended delicate imperfections in the artificial teeth or anatomy that may alter the esthetics and/or function of the artificial teeth.        4) Denture base material shrinkage causes internal stresses which are a source of denture fit problems. During the denture base forming part of the process, methylmethacrylate denture base material polymerizes and shrinks rapidly at first, causing internal stress in the denture base, especially in areas of differing thicknesses. After a denture in devested (removed from denture flask), this internal stress may cause dimensional distortion. Although some commercial continuous pressure curing systems (such as the Ivocap® system by Ivoclar Vivodent, Inc., or the Success® Injection System by Dentsply Prosthetics) reduce this stress, methylmethacrylate continues to slowly polymerize over time until residual monomers are virtually eliminated. Thus distortion (resulting in poor fit) can occur after a denture is completed by a dental laboratory and shipped to a dentist.        
This last problem is illustrated in FIG. 13, which shows a cross-section of a denture base 10 taken along the midline thereof. It can be seen that the denture base 10 has regions of different thickness which may result in areas of more shrinkage than other areas during polymerization of the methylmethacrylate. For example, the thicker region 12 may undergo a greater degree of shrinkage (as indicated by large arrows 13), that the shrinkage of the thinner region 14 (as indicated by smaller arrows 15. These areas of shrinkage differential create stress points, such as stress point 16. Such stress points may cause dimensional distortion of the denture base, resulting in the denture having a poor fit to the patient's mucosa.
The dental laboratory industry in the United States is currently undergoing some changes away from “lost-wax” techniques to computer-aided-design (CAD) and computer-aided-manufacturing (CAM) of dental products. Dental crowns and bridges have been milled using CAD-CAM technologies for many years. The use of these technologies for crowns and bridges has been increasing rapidly in recent years. However, there has been very little use of CAD-CAM technologies in the area of removable prosthodontics (e.g., dentures, partials, etc.).
There are several reasons why CAD-CAM has not been more widely used in removable prosthodontics. A major reason is that satisfactory software for occlusion (i.e., mapping of how upper and lower teeth work together) has not been developed. Another reason is because a denture is made from two colors of materials (a pink base and white teeth), and dental milling systems are based on monolithic materials. Additional reasons are described in the Applicant's commonly owned U.S. Pat. No. 8,641,938 of Howe for a “Denture and Method and Apparatus of Making Same,” the disclosure of which is incorporated herein by reference.
What is needed is a simple, low cost method of manufacturing a denture in which the denture is accurately formed, dimensionally stable, and robust, i.e., resistant to loosening and/or loss and/or fracture of teeth. Additionally, there is a need for a method of making a denture base disc as a starting material for a denture, which is free of stresses and other defects that result in making dentures that do not meet quality requirements.